Dysregulation of members of the epidermal growth factor (EGF) receptor (EGFR) family is associated with oncogenesis and tumor growth. Due to its relevance in cancer development, EGFR is an important target in cancer drug discovery, and several EGFR targeted therapies have already been developed. Their clinical success has, however, often been modest, reinforcing the need for a more complete understanding of the EGFR signaling pathway. This proposal focuses on the insufficiently understood role of large-scale EGFR associates (clusters) in signaling initiation and transduction, in particular. While i has long been known that ligand induced dimerization plays a critical role in receptor signaling, there is growing evidence that this textbook model needs to be augmented to account for the heterogenous lateral distribution of the receptor in the plasma membrane. The local enrichment of the receptors in ''micro-domains or ''nanoclusters'' could strongly affect cooperative receptor interactions and shift the local EGFR association equilibria through a local concentration effect. The experimental investigation of the fundamental mechanisms underlying the large-scale receptor organization with conventional fluorescence microscopy remains challenging, due to the method's limitation with regard to throughput, spatial and temporal resolution, and maximum observation time. Plasmon Coupling Microscopy (PCM) is a novel non-fluorescence based approach that uses electromagnetic interactions between noble metal nanoparticles (NPs) to investigate receptor clustering on subdiffraction limit distances (but beyond the spatial barrier o Fluorescence Resonance Energy Transfer, FRET). NP's do not blink or bleach, are very bright, and can be imaged in a conventional widefield microscope. Consequently, PCM facilitates the monitoring of EGFR clustering without limitations in observation time in many individual cells simultaneously. This competitive renewal builds upon the plasmon coupling based tools developed in the previous funding cycle and outlines a vigorous research plan to elucidate the structural origin of dynamic EGFR clustering. PCM will then be applied to test the hypothesis that receptor clustering regulates the mode and strength of signaling and to elucidate the mechanisms underlying a spatial regulation of signaling intensity and outcome. The obtained insight will improve the understanding of spatial regulation mechanisms for a broad range of receptors. Noble metal NPs are not only superb optical labels for characterizing EGFR clustering in the plasma membrane, but they also represent potential therapeutic tools to restore and enhance negative EGFR signaling after covalent attachment to EGF. This hypothesis is experimentally tested in this proposal. If successful, this strategy would provide a new approach for overcoming apoptosis evasion in cancer.